We have built a "gentle" fast-freezing device and have applied it to isolated stacks of Torpedine ray electric organ. The tissues have been freeze-substituted and the nerve terminal ultrastructure was examined by transmission electron microscopy. About 1 in 3 terminal cross sections shows rows of synaptic vesicles aligned along thge terminal membrane in apparent "active zones". We are proposing to utilize the inherent efficiency of the ray electric organ for morphometric analysis in order to: (1) Determine whether a single deplorizaion of undrugged tissue causesflattening of vesicles at active zones or whether it initiates vesicle recycling so as to generate a locally and preferentially reutilized pool. (2) To follow the effects of trains of stimuli and drugs on the behavior of vesicles in the "active zones", including their restocking by localvesicle recycling or by recuritment from the cytoplasmic pool.